Timer Jitter when 5v not applied to VBUS

[dlktdr]

Hello,

I'm having an issue I hope someone might be able to help with. I have a NRF52840 (Nina B306) / Arduino Nano 33BLE device. Not sure if it's an actual issue or just something I'm doing wrong.

I'm using TIMER3 GPIOTE7 and PPI19 to generate a PWM signal for an RC transmitter. I'm getting a decent amount of jitter ~= 18us on the timers compare. If the USB is attached (5v applied to VBUS) it is nearly perfect, apx 1.5us jitter. The only thing I can see different when applying power to the VBUS pin is that it doesn't go to sleep when 5v is applied.

I'm not concerned with the power requirements for this and was trying to disable the Power management states, following the guide in \samples\boards\nrf\system_off but haven't had any luck. Still as soon as the USB is disconnected the jitter re-appears.

Below is the code I have tried.

static int disable_ds_1(const struct device *dev)
{
	ARG_UNUSED(dev);

    // Disable all power states that may cause interrupt timing issues
	pm_ctrl_disable_state(PM_STATE_SOFT_OFF);
    pm_ctrl_disable_state(PM_STATE_SUSPEND_TO_RAM);
    pm_ctrl_disable_state(PM_STATE_SUSPEND_TO_IDLE);
    pm_ctrl_disable_state(PM_STATE_SUSPEND_TO_DISK);
    pm_ctrl_disable_state(PM_STATE_RUNTIME_IDLE);
    pm_ctrl_disable_state(PM_STATE_STANDBY);

	return 0;
}
SYS_INIT(disable_ds_1, PRE_KERNEL_2, 0);

prj.conf

CONFIG_PM=y
CONFIG_PM_STATE_LOCK=y

It made no difference.

Am I looking at the incorrect cause of this issue? or disabling the power states incorrectly?

Thank you for any help. PPM Code is here https://github.com/dlktdr/HeadTracker/blob/master/firmware/src/src/PPM/PPMOut.cpp

Some oscilliscope screenshots of the jitter are here, dlktdr/HeadTracker#49

Thank you for any insight!

[dlktdr]

Nevermind found it..

When VBUS is at +5v it also always enables the accurate HF clock source. I set it to always HF and works like I was hoping!

    // Force High Accuracy Clock
    const char *clock_label = DT_LABEL(CLOCK_NODE);
	clock0 = device_get_binding(clock_label);
	if (clock0 == NULL) {
		printk("Failed to fetch clock %s\n", clock_label);
	}
    clock_control_on(clock0,CLOCK_CONTROL_NRF_SUBSYS_HF);